1. Field of the Invention
The present invention relates to a method of producing an active material used for a lithium secondary battery, a method of producing an electrode for a lithium secondary battery, a method of producing a lithium secondary battery, and a method of monitoring a quality of an active material for a lithium secondary battery, and is particularly characterized in that, by cleaning an active material including a lithium transition metal oxyanion compound with a pH buffer solution, the amounts of impurities in the active material is reduced, and higher quality and enhancement of an energy density are realized.
2. Description of the Related Art
With respect to a nonaqueous electrolyte secondary battery, generally at present, LiCoO2 is used for a positive electrode, and a lithium metal, a lithium alloy, or a carbon material capable of storing and releasing lithium is used for a negative electrode, and an electrolyte made of lithium salt such as LiBF4 or LiPF6 is used as a nonaqueous electrolyte solution by dissolving in an organic solvent such as ethylene carbonate or diethyl carbonate. However, when LiCoO2 is used for a positive electrode, a production cost becomes high since the reserves of metal Co is limited and metal Co is rare resources. Further, a battery using LiCoO2 has a problem that a battery in a charge state is very low in heat stability at elevated temperatures compared with that in a normal usage state. Therefore, use of LiMn2O4 or LiNiO2 is investigated as a positive electrode material instead of LiCoO2, but LiMn2O4 is not expected to have a sufficient discharge capacity and further has a problem that manganese is dissolved when a battery temperature is elevated. On the other hand, LiNiO2 has problems that a discharge voltage becomes low etc.
In recent years, olivine type lithium phosphate such as LiFePO4 attracts attention as a positive electrode material instead of LiCoO2. Olivine type lithium phosphate is a lithium complex compound expressed by the general formula LiMPO4, wherein M represents at least one element selected from Co, Ni, Mn, and Fe, and its working voltage varies depending on a species of a core metal element M. And, the lithium complex compound has advantages that a battery voltage can be freely selected depending on the selection of an element M and a battery capacity per unit weight can be increased since a theoretical capacity is as relatively high as about 140 to 170 mAh/g. Furthermore, it is possible to select iron as M in the general formula. Since iron has a large output and is inexpensive, iron has an advantage that a production cost can be significantly reduced by use itself, and it is suitable for a positive electrode material of a large-scale battery or a high-power battery.
As a synthetic method of LiFePO4, various synthetic methods such as a solid-phase process, a hydrothermal process and a coprecipitation process are proposed. In Patent Publication No. 3484003, a reaction of Li2CO3+2FeC2O4.2H2O+2(NH4)2HPO4→2LiFePO4+4NH3+5CO2+5H2O+2H2 is used to synthesize LiFePO4 in a solid-phase process. And, in Japanese Patent Laid-Open No. 2002-110162, a reaction of Li3PO4+Fe3(PO4)2.nH2O→3LiFePO4+nH2O is used to synthesize LiFePO4 in a solid-phase process.
However, in these synthetic methods, when mixing is insufficient or a reaction is not homogeneous, Li2CO3 or Li3PO4 of a raw material remains unreacted and remains as an impurities in an active material.
Since such impurities do not contribute to a charge-discharge reaction, this causes a battery capacity to decrease and further causes internal short-circuit. And, there is a problem that when the amounts of impurities contained in LiFePO4 varies from production lot to production lot, the capacity of a battery prepared by use of this LiFePO4 varies. Furthermore, there are problems that when an impurities exhibits alkalinity, since it reacts with polyvinylidene difluoride (PVdF) which is generally often used as a binder in preparing an electrode, a slurry property in preparing a positive electrode plate is deteriorated to make it difficult to prepare the electrode and electrode strength becomes insufficient.
As a means for eliminating the impurities in lithium complex oxides, cleaning of lithium complex oxide with water is proposed in Japanese Patent Laid-Open No. 2003-17054. As a means for eliminating the impurities in LiFePO4, a method, in which a reaction of FeSO4.7H2O+H3PO4+3LiOH.H2O→LiFePO4+Li2SO4+11H2O is used to mix a raw material, and LiFePO4 is synthesized by a hydrothermal process and then cleaned with distilled water to produce LiFePO4, is proposed in International Publication WO 2005/051840A1 pamphlet.
For a problem that the amounts of impurities contained in LiFePO4 varies from production lot to production lot, a method of monitoring the conductivity of distilled water used in cleaning is proposed in International Publication WO 2005/051840A1 pamphlet, and a method of quantifying the amounts of impurities by a X-ray diffraction method is proposed in Japanese Patent Laid-Open No. 2002-117847.